This application is a 371 of PCT/JP98/03292 filed Jul. 23, 1998.
The present invention relates to a vinylidene fluoride copolymer providing a polymer matrix for forming a gel-form solid electrolyte suitable for forming a non-aqueous battery, particularly a lithium ion battery, and a gel-form solid electrolyte formed of the vinylidene fluoride copolymer and a non-aqueous battery comprising the solid electrolyte.
The development of electronic technology in recent years is remarkable, and various apparatus and devices have been reduced in size and weight. Accompanying the reduction in size and weight of such electronic apparatus and devices, there has been a remarkably increasing demand for reduction in size and weight of a battery as a power supply for such electronic apparatus and devices. In order to generate a larger energy from a battery of small volume and weight, it is desirable to generate a higher voltage from one battery. From this viewpoint, much attention has been called to a battery using a negative electrode substance comprising, e.g., lithium or a carbonaceous material capable of being doped with lithium ions, and a positive electrode active substance comprising, e.g., a lithium-cobalt oxide.
However, in case where an aqueous electrolytic solution is used, it is easily decomposed in contact with lithium, a carbonaceous material doped with lithium ions or a lithium aluminum alloy, so that a non-aqueous electrolytic solution formed by dissolving a lithium salt in an organic solvent has been used as the electrolytic solution. As the electrolyte for such a non-aqueous electrolytic solution, there are known LiPF6, LiAsF6, LiClO4, LiBF4, LiCH3SO3, LiCF3SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiCl, LiBr, etc. Further, as the organic solvent for the electrolyte, there is principally used a solvent mixture of a solvent having a high dielectric constant and well dissolving the electrolyte, such as propylene carbonate, ethylene carbonate or xcex3-butyrolactone, and a low-boiling point solvent, such as 1,2-dimethoxyethane, 1,2-diethoxyethane, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propionate or ethyl propionate. The solvent having a high dielectric constant generally has a high boiling point of ca. 200xc2x0 C. or higher and a low vapor pressure at ordinary temperature, whereas most low viscosity solvents generally have a boiling point around ca. 100xc2x0 C. and a high vapor pressure at ordinary temperature.
On the other hand, in case where such a non-aqueous secondary battery filled with an organic electrolytic solution is exposed to a high temperature causing a very high vapor pressure of the electrolytic solution inside thereof or excessively charged to generate a decomposition gas of the electrolytic solution, a dangerous state of causing an increase in battery internal pressure possibly leading to an explosion is expected. For this reason, currently commercially available non-aqueous secondary batteries are equipped with a rupture plate for releasing an excessively high pressure before explosion of the battery per se. The operation of the rupture plate results in leakage of a readily ignitable organic electrolytic solution outside the battery. Such leakage of the electrolyte may presumably be also caused by a deterioration with time of a packing between the can body and the cap or a deformation of the packing due to careless handling of the battery. Accordingly, a battery using a non-aqueous electrolyte involves a potential risk of a fire in case of leakage of the non-aqueous electrolytic solution outside the battery by any chance due to a high pressure and ready ignitability of the electrolytic solution.
Non-aqueous lithium-based secondary batteries have been heretofore principally used as power sources for home-use small-capacity electronic appliances, such as portable telephone sets, personal computers and video camera-covers. Heretofore, no fire accident has been caused at all on the market in ordinary environments of use, and general understanding has been attained regarding the safeness of secondary batteries. Accordingly, based on such actual results of safety, the development of secondary batteries as large electricity sources, such as those for electromotive vehicles and load leveling for effective utilization of night electricity, has recently become earnest. As the batteries become larger, the risk of an accidental fire becomes larger to an extent beyond comparison with that in the case of small-capacity batteries.
The present inventors have studied for improvement of problems regarding the safeness of a secondary battery, while noting that such problems are attributable to the use of an organic solvent, particularly a low-viscosity solvent having a high vapor pressure at low temperatures and the structure wherein the organic electrolytic solution is readily leaked out on an occasion of mal-function of the packing of the battery caused by any chance. Accordingly, it has been considered essential to use solid polymer electrolytes, inclusive of, e.g., one formed by dispersing a lithium electrolyte, such as LiClO4 or LiPF6 in a gel-form substance composed of polyethylene oxide as a polymer and propylene carbonate as a highly dielectric solvent, developed since 1970""s. Several solid polymer electrolytes have been reportedly developed, and actually primary batteries using them have been commercialized. However, no secondary batteries having a cycle characteristic of more than several hundred cycles, have been realized. One cause thereof may be the reduction of the polymer matrix substance used for the solid electrolyte at the boundary with a negative electrode of lithium metal or doped with lithium resulting in a growth of a passive state film showing a poor conductivity for lithium ions. Another cause may be the use of a solid polymer electrolyte showing a lower conductivity for lithium ions than a conventional electrolytic solution using an organic solvent, thus resulting in a battery having a high internal resistance, whereby the utilization of a full capacity of the electrode active substance is liable to cause excessive charging and excessive discharging, thus leading to a deterioration of the electrode active substance in a short period.
By the way, vinylidene fluoride polymer is currently extensively used as a binder for binding an electrode active substance in small-capacity lithium ion secondary batteries using non-aqueous electrolytic solutions. This is because the vinylidene fluoride polymer is not at all reduced in a reducing atmosphere on a negative electrode where tetrafluoroethylene polymer is readily reduced, or is not at all oxidized in an oxidizing atmosphere on a positive electrode where most organic electrolytic solutions are oxidized, so that it is electrochemically stable over a wide potential window.
Further, vinylidene fluoride monomer has two hydrogen atoms functioning as electron donors, and two fluorine atoms functioning as electron acceptors, and therefore has a high polarization as a monomer unit so that it functions as a medium capable of well dissolving therein polar substance, such as an electrolyte.
As has been clarified in Japanese Patent Publication (JP-B) 54-044220, it is known that even a macromolecule such as an organic dye molecule can be migrated at a high speed within a polymer at room temperature if the polymer has a low glass transition temperature. Vinylidene fluoride polymer has a glass transition temperature as low as xe2x88x9245xc2x0 C., which means that room temperature is higher than its glass transition temperature by more than 50xc2x0 C., so that the molecular movement at an amorphous portion thereof is sufficiently active and it is considered to exhibit a capability of transporting an electrolyte contained therein at a high speed.
For the above-mentioned reasons in combination, vinylidene fluoride polymer is considered to be extensively used as a binder which is required to satisfy mutually contradictory properties that it encloses an electrode active substance and it is free from obstruction of transportation of lithium ions to the interior of the active substance.
In view of the above, it may well be expected to use vinylidene fluoride polymer for constituting a basic matrix of a solid polymer electrolyte. This has been already reported in Japan in early 1980""s, regarding a solid polymer electrolyte using vinylidene fluoride polymer (Tsuchida, E., et al.; Electrochimica Acta. 28 (5), 591-595 (1983)).
However, vinylidene fluoride polymer is a crystalline polymer having a crystallinity of ca. 50%, so that the ionic conductivity at a crystalline portion is considered to be very low because of extremely poor molecular mobility at a crystalline portion of a polymer. For this reason, in 1990""s, a solid polymer electrolyte using a copolymer of vinylidene fluoride and hexafluoropropylene having a lower crystallinity has been reported as disclosed in U.S. Pat. No. 5,296,318. The vinylidene fluoride copolymer copolymerized with 8 wt. % or more of hexafluoropropylene has a very low crystallinity because trifluoromethyl groups in the hexafluoropropylene provide steric hindrance, so that it is considered to have provided a higher ionic conductivity than in the one using vinylidene fluoride homopolymer.
However, it has become clear that the solid polymer electrolyte using vinylidene fluoride-hexafluoropropylene copolymer involves a serious defect for practical use. More specifically, when a gel is formed as a mixture thereof with an organic solvent to be used as a material for a secondary battery, the gel exhibits insufficient adhesiveness onto an electrode substrate as represented by a copper foil (for a negative electrode) or an aluminum foil (for a positive electrode), thus being liable to result in a peeling of the gel layer containing powdery electrode materials, such as an active substance, leading to practical problems, such as a lowering in production yield during battery assembling steps and a lowering with time of discharge capacity of a battery during a long period of use of the battery.
When the adhesiveness of a gel electrode swollen with an electrolytic liquid onto a smooth surface like that of a metal foil is considered, the electrolytic liquid contained in the gel inevitably obstructs the chemical interaction between the gel and the electrode substrate to result in a lower adhesion in case of using such a copolymer. On the other hand, there has been proposed an idea of providing unevennesses to an electroconductive substrate surface by etching, etc., so as to physically bond the gel due to the anchoring effect. This requires a troublesome surface treatment of the electroconductive substrate and yet cannot necessarily fulfill a sufficient effect as expected.
A principal object of the present invention is to provide a vinylidene fluoride copolymer suitable for forming a polymer matrix providing a solid polymer electrolyte which exhibits an appropriate level of ionic conductivity in its state of being swollen with a non-aqueous electrolytic solution, excellent adhesion to an electroconductive substrate and retentivity of powdery electrode materials, and further excellent heat resistance.
Another object of the present invention is to provide a solid polymer electrolyte formed by using such a vinylidene fluoride copolymer and a non-aqueous battery using the solid polymer electrolyte.
According to the inventors study, it has been found very effective for accomplishing the above objects to use a vinylidene fluoride copolymer having a moderately reduced vinylidene fluoride content and an increased amorphous content, having an appropriate polar group introduced by copolymerization and having been crosslinked.
Thus, according to a first aspect thereof, the present invention provides a vinylidene fluoride copolymer comprising 50-97 mol. % of vinylidene fluoride monomer, and 0.1-5 mol. % of a monoester of unsaturated dibasic acid or an epoxy group-containing vinyl monomer, and having been chemically or physically crosslinked.
The improved adhesion with the electroconductive substrate and retentivity of powdery electrode materials are considered to have been attained by the introduction of a polar group comprising the acid or epoxy group and a rubbery characteristic caused by an increased amorphous portion due to a lower vinylidene fluoride content.
Further, the solid polymer electrolyte of the present invention is characterized by comprising a polymer matrix comprising the above-mentioned vinylidene fluoride copolymer, and a non-aqueous electrolytic solution impregnating the polymer matrix.
The present invention further provides a solid polymer electrolyte for forming a secondary battery equipped with a positive electrode comprising a positive electrode material capable of being doped with and liberating lithium, and a negative electrode comprising a negative electrode material capable of similarly being doped with and liberating lithium, wherein an electrode structure is formed by binding and retaining a powdery electrode material for constituting the positive electrode or the negative electrode with the above-mentioned crosslinked vinylidene fluoride copolymer and is impregnated with a non-aqueous electrolytic solution to render the vinylidene fluoride copolymer to be a gel-form solid electrolyte so as to form an electrode structure integral with the powdery electrode material.
The present invention further provides a non-aqueous battery comprising a positive electrode, a negative electrode, and any solid polymer electrolyte mentioned above disposed between the positive and negative electrodes.
More specifically, a solid polymer electrolyte formed by impregnating the polymer matrix comprising the above-mentioned crosslinked vinylidene fluoride copolymer and containing substantially no powdery electrode material, when placed between a pair of the positive electrode and the negative electrode, functions as both an electrolytic solution and a separator.
Further, a solid polymer electrolyte layer formed by dispersing a positive electrode material or a negative material in the solid polymer electrolyte functions as a positive electrode layer or a negative electrode layer, respectively.
Then, a non-aqueous battery according to the present invention may be formed by disposing the polymeric solid electrode layer functioning as a separator sandwiched between the solid polymer electrolyte layers as a positive electrode layer and a negative electrode layer respectively bonded to electroconductive substrates. The solid polymer electrolyte constituting the positive electrode layer and the negative electrode layer, and the solid polymer electrolyte also functioning as a separator, are all formed of gels, so that they exhibit a good adhesion with each other and provide a structure of laminated layers which are not readily peeled from each other.
Further, it is also possible to use such positive and negative electrode structures comprising the gel-form solid electrolytes in a conventional battery using a non-aqueous electrolytic solution and a separator and not using such a solid polymer electrolyte layer comprising a polymer retaining a non-aqueous electrolytic solution.